Mounting means for thermal switches

ABSTRACT

A mounting device for conveniently and inexpensively mounting thermal switches is provided, comprising (a) an annular support means of heat-conductive material adapted to be positioned coaxially around the switch, and (b) a fastening means of heatconductive material extending integrally from the annular support means for attaching the annular support means to a heattransmitting part of an electrical device whose temperature is to be sensed by the thermal switch.

Uited States Patent Ball [11] 3,827,015 July 30, 1974 122 Filed:

[ MOUNTING MEANS FOR THERMAL SWITCHES [75] Inventor: James H. Ball, Saint Paul, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

Apr. 13, 1973 [21] Appl. No.: 350,740

US. Cl. 337/414, 337/208 [51] Int. Cl. HOlh 37/04 Field of Search 337/398, 186, 199, 207, 337/208, 414; l74/D1G. 5; 338/315;

[56] References Cited UNITED STATES PATENTS 1,482,032 1/1924 Pawlik ct a1. 337/414 10/1942 Godscy, Jr. 317/242 8/1948 Antalek 317/242 Primary Examiner-Harold Broome orn ent. e .firmr za nsn llz-firdsiti De LaHunt 5 7] ABSTRACT A mounting device for conveniently and inexpensively mounting thermal switches is provided, comprising (a) an annular support means of heat-conductive material adapted to be positioned coaxially around the switch, and (b) a fastening means of heat-conductive material extending integrally from the annular support means for attaching the annular support means to a heattransmitting part of an electrical device whose temperature is to be sensed by the thermal switch.

12 Claims, 4 Drawing Figures MOUNTING MEANS FOR THERMAL SWITCHES BACKGROUND OF THE INVENTION Thermal switches, in which an electrical path normally extending through the switch is interrupted when the switch is subjected to a predetermined temperature, are included in a variety of electrical devices as protection against malfunctions that would otherwise drive the devices to an excessive temperature condition. Some especially useful thermal switches are miniature cylindrical switches of the type taught in US. Pat. No. 3,180,958, in which a solid pellet normally holds two electrical contacts together against spring pressure that would separate the contacts; when the pellet is subjected to a precise predetermined temperature, it melts, and the spring pressure separates the contacts.

To enable the thermal switch to function reliably and quickly, heat produced by a malfunction must be quickly and uniformly transported to the switch. In

many appliances and devices, the transport of heat from a malfunctioning part-by conduction through parts near or contiguous to the switch, by conduction through the circuit containing the switch, by radiant energy, by IgR heat effects in the circuit, and by convection is insufficient for quick and reliable operation of the switch. The transport of heat under these circumstances is too slow and it is sometimes unpredictable, with the result that the switch fails to perform as desired.

In one arrangement that has been used commercially to provide a more rapid and uniform transport of heat to a thermal switch, the switch is enclosed within a housing that is adapted to be mounted, as by screws that extend through flanges on the housing, on a part that will become heated during a malfunction of the device to be protected by the switch. The housing is generally plastic, except that a wall of the housing that is placed in direct contact with the part that will become heated consists of a heat-conductive metal. A layer of electrical insulation covers the inside of the metal wall, and a block of heat-conductive metal rests against the layer of insulation. The block of metal contains a small cylindrical recess into which the switch fits, so that heat is conducted through the metal wall from the heated part, through the layer of insulation, and through the block of metal to the switch. Two metal leads extend from the switch through the plastic housing for electrical connection of the switch into the circuit to be protected.

While such a mounting device will provide a more uniform and rapid heating of the thermal switch than previous mounting arrangements provide, use of such a mounting device is undesirable for several reasons. First, such a mounting device is far too expensive; the cost of manufacturing and assembling the mounting device can be as high as the cost of the thermal switch itself. Further, the bulky housing curtails advantages obtained by the miniature size of the thermal switch. Also, the heat-transmitting part to which the mounting device is to be attached is not always adapted for such an attachment: the heat-transmitting part does not always have a flat surface for contact with the flat metal wall of the housing; and the mounting device is sometimes larger than the heat-transmitting part. For all these reasons, there exists a need for better mounting arrangements for thermal switches in many electrical devices.

SUMMARY OF THE INVENTION Briefly, according to the present invention, thermal switches are generally mounted in heat-conducting relation in a mounting means that comprises (a) an annular support means of heat-conductive material adapted to be positioned coaxially around the switch, and (b) a fastening means of heat-conductive material extending integrally from the annular support means for attaching the annular support means to a heattransmitting part of an electrical device whose temperature is to be sensed by the thermal switch.

Such a mounting means is of the utmost simplicity to manufacture; a thermal switch is easily mounted in the mounting device; the mounting device may easily be attached to a heat-transmitting part; and heat is reliably and quickly transmitted to the thermal switch inside the mounting device.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an illustrative mounting device of the invention, in which a thermal switch surrounded by a tube of electrical insulation is mounted;

FIG. 2 is a section through the combination shown in FIG. 1 taken along the lines 2-2 of FIG. 1;

FIG. 3 is a perspective view of a different embodiment of mounting device of the invention; and

FIG. 4 is a generally schematic view of an electrical device incorporating a mounting device of the invention.

DETAILED DESCRIPTION The mounting device 10 shown in FIGS. 1 and 2 comprises a sheet of heat-conductive material such as aluminum or tin-plated brass. The center portion of the sheet has been shaped as an annular support means 11 that holds a thermal switch 12 that is covered by a preformed tube 13 of electrical insulation, and the two ends of the sheet have been juxtaposed against one another to form a fastening means in the form of a platelike member 14 that is integral with the annular support means. The plate-like member 14 is apertured at E5 to receive a fastener such as a screw or bolt by which the mounting device can be fastened to a heat-transmitting part in a device whose temperature is to be sensed.

As shown in FIG. 2, the illustrative thermal switch 12 comprises an outer electrically conductive casing 17, a ceramic support 18 within and at one end of the casing in which a first electrode 19 is held, an outer sealant 20, a second electrode 21 fastened to the other end of the casing 17, a pellet 22 of thermally softenable material, and a contact disc 23 having finger-shaped extensions that engage the casing; the disc 23 is held against the electrode 19 by a compression coil spring 24 acting on two load-distributing protective plates 25 and 26. When the thermal switch 12 is subjected to a predetermined temperature, the pellet 22 collapses and flows around the plate 25, whereupon the force of spring 24 is released. A second spring 27, relatively weaker than spring 24, then urges the contact disc 23 away from the end of electrode 19. It may be noted that the thermal switch 12 is only illustrative, and any thermal switch requiring a reliable and uniform transport of heat to it is used in the invention.

In some useful mounting devices of the type shown in FIGS. 1 and 2, one end of the sheet of thermally conductive material is shaped as the annular support means and the other end comprises a plate-like member. The convenience and economy of manufacture of such a device is an important advantage. However, mounting devices in which the two ends are juxtaposed also have an advantage, in that they permit a clamping action when a screw-type fastener is tightened into the heattransmitting part that is to carry the mounting device.

Preferably, the sheet of heat-conductive material from which a mounting device as shown in FIGS. 1 and 2 is formed consists of a rather soft confonnable metal to permit good contact between the mounting device, electrical insulation, and thermal switch. Also, so that the mounting device can be welded to a heattransmitting part in which the thermal switch is to be placed, the plate-like member is desirably made of a metal to which welded bonds may be made. The platelike member should not warp and separate from the heat-transmitting part to which it is attached. The plate-like member is normally rather thin, generally between about 25 and 75 mils thick. It need not be flat, as when it is to be attached to a curved part. In some instances the fastening means in a mounting device of the invention comprises a cylindrical extension of the annular support means that, for example, fits snugly into a socket of a heat-transmitting part.

While the heat-conductive material is usually metal, if a slower response by the thermal switch is desired (as where ,a heating device in normal operation experiences temperature overshooting that should not be sensed by the thermal switch) or if a slower response can be tolerated, a dielectric material such as nylon or polytetrafluoroethylene is used. In such an arrangement a separate layer of electrical insulation need not be placed between the thermal switch and annular support means.

The preformed tube 13 of electrical insulation used in the embodiment of FIGS. 1 and 2 not only serves as electrical insulation but also improves heatconductivity, since it fills the space between the annular support means II and the thermal switch 12. Sometimes its only function is to fill this space, since the thermal switch and mounting device may be both connected to ground, for example. Organic polymeric materials, such as silicone rubber, polyvinyl chloride, or polyethylene terephthalate, (often coated over a fibrous glass sleeve) are generally used as the material in the space between the annular support means and thermal switch. Generally the more conformable the material, the better; and elastomeric materials are often preferred. Instead of using a preformed tube of insulation over the thermal switch, a tape may be wrapped around the thermal switch; or the annular support means of the mounting device is, in some embodiments of the invention, coated with an organic polymeric material.

FIG. 3 shows a different embodiment of the invention in which adhesive is used to join the mounting device to a heat-transmitting part of a device whose temperature is to be sensed. A mounting device 29 as shown in FIG. 3 comprises a planar sheet 30 of heat-conducting material from which an annular support means 31 has been cut and shaped. A layer 32 of heat-conducting adhesive is disposed on the bottom surface of the planar sheet 30. The adhesive layer should be thin, generally less than about 5 mils and preferably less than about 2 mils thick, to permit good heat-conduction. Generally, thermosetting adhesives, such as epoxy-, acrylate-, or silicone-based adhesives, are used.

The embodiment shown in FIG. 3 is adapted to be supplied as part of a set. Such a set generally comprises mounting devices 29 joined end-to-end along lines of weakness 33 formed by partially cutting a base sheet from which the set of mounting devices is formed.

The heat-transmitting part to which a mounting device of the invention is attached may take various forms. In many situations it is the casing of a part that may malfunction. In other situations it is a part that is near a part that may malfunction. FIG. 4 illustrates one arrangement, showing in a generally schematic manner an electrical device 35 having a plastic housing 36 enclosing a heating coil 37 mounted above a bottom wall 38. A mounting device 39 of the invention supporting a thermal switch 40 is carried on the housing 36. A bolt 41 having a large head 42 on the inside of the housing 36 extends through an aperture in a plate-like member of the mounting device 39 and a nut 43 screws onto the bolt 41 to hold the mounting device in place. In this embodiment the heat-transmitting part comprises the bolt 41, which is exposed to the heat of the heating coil and transmits that heat to the mounting device. If a malfunction causes the heating coil to be heated to an unduly high temperature, the thermal switch will sense that excess heat by transmission through the heattransmitting bolt 41 and the mounting device 39.

I claim:

1. In combination,

1. a cylindrical thermal switch and, in heatconducting relation to the thermal switch,

2. a mounting device comprising (a) an annular support means of heat-conductive material positioned coaxially around the switch, (b) fastening means of heat-conductive material extending integrally from the annular support means for attaching the annular support means to a heat-transmitting part of an electrical device whose temperature is to be sensed by the thermal switch, and (c) a layer of heatconducting organic polymeric material filling the space between the annular support means and the switch.

2. A combination of claim 1 in which the fastening means comprises a plate-like member.

3. A combination of claim 2 in which the plate-like member is apertured to receive a fastener that is adapted to extend into said heat-transmitting part.

4. A combination of claim 2 in which the plate-like member carries a layer of adhesive for attaching the member to said heat-transmitting part.

5. In combination,

1. a cylindrical thermal switch and, in heatconducting relation to the thermal switch,

2. a mounting device for the switch comprising a sheet of metal shaped to provide (a) an annular support means arranged coaxially around the switch and (b) a plate-like member integral with the annular support means for attaching the annular support means to a heat-transmitting part of an electrical device whose temperature is to be sensed by the thermal switch; and a layer of heatconducting organic polymeric material filling the space between the annular support means and the switch.

6. A combination of claim 5 in which the layer of electrical insulation comprises a tube frictionally fit around the thermal switch.

7. A combination of claim 5 in which the layer of electrical insulation comprises a coating on the annular support means.

8. In combination,

1. an electrical device having at least one component that can heat to an overheated condition as a result of electric power supplied to the device;

2. a cylindrical thermal switch electrically connected into the electrical circuit that supplies electric power to the electrical device, said thermal switch opening to an open-circuit condition when it reaches a predetermined temperature; and, in heatconducting relation to the thermal switch,

3. a mounting device comprising (a) an annular support means of heat-conductive material positioned coaxially around the switch, and (b) engaging means of heat-conductive material extending integrally from the annular support means and into engagement with a heat-transmitting part of said electrical device so as to provide a path for heat to travel from said component to said thermal switch.

switch. 

1. In combination,
 1. a cylindrical thermal switch and, in heat-conducting relation to the thermal switch,
 2. a mounting device comprising (a) an annular support means of heat-conductive material positioned coaxially around the switch, (b) fastening means of heat-conductive material extending integrally from the annular support means for attaching the annular support means to a heat-transmitting part of an electrical device whose temperature is to be sensed by the thermal switch, and (c) a layer of heat-conducting organic polymeric material filling the space between the annular support means and the switch.
 2. a mounting device comprising (a) an annular support means of heat-conductive material positioned coaxially around the switch, (b) fastening means of heat-conductive material extending integrally from the annular support means for attaching the annular support means to a heat-transmitting part of an electrical device whose temperature is to be sensed by the thermal switch, and (c) a layer of heat-conducting organic polymeric material filling the space between the annular support means and the switch.
 2. A combination of claim 1 in which the fastening means comprises a plate-like member.
 2. a mounting device for the switch comprising a sheet of metal shaped to provide (a) an annular support means arranged coaxially around the switch and (b) a plate-like member integral with the annular support means for attaching the annular support means to a heat-transmitting part of an electrical device whose temperature is to be sensed by the thermal switch; and a layer of heat-conducting organic polymeric material filling the space between the annular support means and the switch.
 2. a cylindrical thermal switch electrically connected into the electrical circuit that supplies electric power to the electrical device, said thermal switch opening to an open-circuit condition when it reaches a predetermined temperature; and, in heat-conducting relation to the thermal switch,
 3. a mounting device comprising (a) an annular support means of heat-conductive material positioned coaxially around the switch, and (b) engaging means of heat-conductive material extending integrally from the annular support means and into engagement with a heat-transmitting part of said electrical device so as to provide a path for heat to travel from said component to said thermal switch.
 3. A combination of claim 2 in which the plate-like member is apertured to receive a fastener that is adapted to extend into said heat-transmitting part.
 4. A combination of claim 2 in which the plate-like member carries a layer of adhesive for attaching the member to said heat-transmitting part.
 5. In combination,
 6. A combination of claim 5 in which the layer of electrical insulation comprises a tube frictionally fit around the thermal switch.
 7. A combination of claim 5 in which the layer of electrical insulation comprises a coating on the annular support means.
 8. In combination,
 9. A combination of claim 8 in which the engaging means comprises a plate-like member.
 10. A combination of claim 9 in which the plate-like member is apertured to receive a fastener that extends into said heat-transmitting part.
 11. A combination of claim 9 in which the plate-like member carries a layer of adhesive that attaches the plate-like member to said heat-transmitting part.
 12. A combination of claim 8 in which a layer of heat-conducting electrical insulation fills the space between the annular support means and the thermal switch. 